In recent years, various types of rear wheel steering apparatuses forming a portion of a four wheel steering (4WS) system of a vehicle have been known. Typically, as disclosed in JP 7-215226A (Reference 1), a rear wheel steering actuator is “configured to include an electric motor; a speed reduction mechanism that is connected to an output of the electric motor; and a conversion mechanism that converts output rotational motion of the speed reduction mechanism into linear motion, and transmits the linear motion to an axle (refers to Paragraph [0003] of Reference 1)”. The size of the actuator is reduced by inserting and coaxially disposing an actuator shaft into a circular tube-shaped rotator member, and a planetary gear mechanism is used as a speed reduction mechanism for a motor output.
JP 2008-164014A (Reference 2) discloses “an expandable and contractible actuator for the purpose of improving its adaptability for various usages at a low cost (refer to Paragraph [0004] of Reference 2)”. In Reference 2, “a first housing is attachably and detachably provided to accommodate a motor of the expandable and contractible actuator, and a second housing is attachably and detachably provided to accommodate a speed reducer and a feed screw mechanism. In a case where specifications of the motor are desired to be changed, or in a case where operational characteristics of the speed reducer or the feed screw mechanism are desired to be changed, it is possible to cope with this case by only replacing a first housing side sub-assembly or a second housing side sub-assembly without changing the entire design of the expandable and contractible actuator, and it is possible to improve the adaptability of various models and to reduce cost (refer to Paragraph [0011] of Reference 2)”. Also, in the expandable and contractible actuator disclosed in Reference 2, a planetary gear mechanism is used as a speed reduction mechanism for a motor output. In addition, in a wheel steering apparatus disclosed in Japanese Patent No. 5098242 (Reference 3), a speed reduction mechanism is provided which converts rotational motion of a motor into linear motion and transmits the linear motion to a rod. A planetary gear mechanism is used as an example of the speed reduction mechanism.
In each of the apparatuses disclosed in References 1 and 2, a planetary gear mechanism is used as the speed reduction mechanism for a motor output, and typically, there is a limitation to combinations of the number of teeth of a planetary gear, which allows meshing of gears. In a case where the actuator axis is coaxially disposed as disclosed in Reference 1, the number of teeth of the planetary gear is limited by the diameter of an output rod (the actuator shaft disclosed in Reference 1) and the outer diameter of the actuator, and thus, it is difficult to change a reduction gear ratio. In contrast, in a case where output characteristics of the actuator disclosed in Reference 2 are changed, the scale of a change is smaller compared to the actuator disclosed in Reference 1; however, it is necessary to change a sub-assembly including a housing. As described in Reference 1, the actuator disclosed in Reference 2 cannot adopt a configuration in which an output rod extends out from both sides of the housing.
In contrast, in the planetary gear mechanism disclosed in Reference 3, a metal flat head pin is used as a member that rotatably supports a planetary gear (illustrated in FIG. 2 of Reference 3). The flat head pin, whose head portion serves as a retainer which prevents axial movement of the planetary gear in an axial direction, is commercially available. However, there is no general-purpose pin available which can be used in a rear wheel steering apparatus, and it is necessary to prepare a dedicated design component, thereby causing an increase in cost.